The present invention relates to an apparatus for fluidizing a particulate material in a conveying gas for transport and distribution, and, more particularly, to such an apparatus adapted for feeding particulate material in a conveying gas to a fluidized bed furnace.
In a typical present day fluidized bed furnace, particulate fuel, such as coal having a top size ranging from about 3.0 to about 6.5 mm, is typically fed to and combusted within a fluidized bed of similar sized particulate material at relatively low temperatures of 760.degree. C. to 925.degree. C. If the fuel being burned contains sulfur, it is customary that the particulate material making up the bed be comprised of a sulfur absorbent, most commonly limestone, in addition to the particulate fuel.
Fluidizing air, which also serves as combustion air, supplied to the fluidized bed from an air plenum located beneath the bed support plate. The fluidizing air passes upwardly from the air plenum into the fluidized bed through a plurality of holes in the bed support plate at a flow rate sufficiently high to fluidize the particulate material within the fluidized bed.
The performance of a fluidized bed for sulfur capture and fuel combustion is known to be improved by recycling solids separated from the flue gas back to the combustor. Recycling of these solids increases the overall residence time of the solids in the combustor and the concentration of solids in the reaction volume above the bed. The recycle solids are typically fed into the bed in a manner similar to that used for feeding fresh particulate material, such as fuel and sulfur absorbent. However, since recycle solids are usually at an elevated temperature, of a fine particulate size and dry, it is generally more difficult to feed recycle solids in a controlled manner in conventional feeders than it is to feed fresh particulate material.
A number of different approaches have been suggested for feeding fresh or recycled particulate material to the bed, including overbed feed systems and underbed feed systems. One particular underbed feed system suitable for feeding particulate material to a fluidized bed is disclosed in U.S. Pat. No. 4,356,779. As disclosed therein, a fuel feeder is disposed beneath the fluidized bed combustor for entraining fuel in air and then feeding that fuel upwardly into the combustor. The feeder housing defines a chamber which is divided into upper and lower sections by a horizontally disposed perforated distributor plate. The particulate fuel to be supplied to the fluidized bed is fed into the upper chamber above the perforated distributor plate while conveying air is supplied to the lower chamber beneath the perforated distributor plate. The air supplied to the lower chamber passes upwardly through the perforated distributor plate to fluidize and entrain the particulate coal in the upper region of the chamber. The entrained coal is then carried upwardly from the chamber to the fluidized bed boiler through transport lines which open to the fluidizing chamber through the roof of the feeder. In the feeder disclosed in U.S. Pat. No. 4,356,779, there is no provision to permit control of the output of particulate material through each of the individual transport lines. Assuming that the lengths of the transport lines from the feeder to its endpoint destination are equal, the output of particulate material from the disclosed feeder would be necessarily evenly distributed amongst the various transport lines leading from the feeder. No provision is made to permit an uneven or selective distribution of the output or to compensate for an inherent uneven distribution of output due to unequal line length.
A feeder apparatus which addresses this disadvantage is disclosed in U.S. Pat. No. 4,530,290 granted to applicant on July 23, 1985. The feeder comprises a housing defining a chamber which is divided by means of a perforated bed support plate, in the form of an inverted truncated cone, disposed thereacross into a gas plenum subadjacent the plate and a particulate fluidizing plenum superadjacent the plate. Gas supply means open into the gas plenum for conveying a pressurized conveying gas therethrough to pass upwardly through the perforated bed support plate to fluidize particulate material deposited in the particulate fluidizing plenum so as to establish a discrete bed of fluidized particulate material superadjacent the plate. Particulate material is fed to the fluidizing plenum through a feed column penetrating the roof of the housing and extending downwardly therein to terminate in the vicinity of the bed support plate so that particulate material is supplied to the chamber at a location beneath the surface of the discrete bed. A plurality of vertically positionable transport conduits extend into the particulate fluidizing chamber so as to open into the splash zone at a desired distance above the surface of the discrete bed to receive particulate material and conveying gas from the splash zone.
One problem encountered in such fludized feeders is sealing the feed column, i.e. the standpipe, through which particulate material is supplied to the feeder from the fluidizing air being supplied to the fludizing chamber to entrain the particulate fuel for transport.
Accordingly, it is an object of the present invention to provide an apparatus for fluidizing a particulate material in a conveying gas wherein the particulate material feed column is inherently sealed from the fluidizing air supplied to the fluidizing chamber.
It is a further object of the present invention to provide such an apparatus wherein the flow of particulate material from the feed column into the bed may be readily controlled, especially for materials such as recycle solids.